The White Sox have used Duke largely in a setup role, not as a lefty specialist. In fact, Duke has limited right-handed batters to a .182 average and .658 OPS, while lefties have hit .264 and posted a .646 OPS against him. Since the start of July, Duke has allowed two earned runs in 13 appearances.
The Cardinals are expected to feature him as a bridge to their setup relievers Kevin Siegrist and Jonathan Broxton. The club has played the majority of the season with one left-hander in the 'pen.
Duke reinvented himself into an effective reliever after a mediocre career as a starting pitcher with the Pirates.